Tunable Self-Referenced Molecular Thermometers via Manipulation of Dual Emission in Platinum(II) Pyridinedipyrrolide Complexes

Optical temperature sensors based on self-referenced readout schemes such as the emission ratio and the decay time are crucial for a wide range of applications, with the former often preferred due to simplicity of instrumentation. This work describes a new group of dually emitting dyes, platinum(II) pincer complexes, that can be used directly for ratiometric temperature sensing without an additional reference material. They consist of Pt(II) metal center surrounded by a pyridinedipyrrolide ligand (PDP) and a terminal ligand (benzonitrile, pyridine, 1-butylimidazol or carbon monoxide). Upon excitation with blue light, these complexes exhibit green to orange emission, with quantum yields in anoxic toluene at 25 °C ranging from 13% to 86% and decay times spanning from 8.5 to 97 μs. The emission is attributed to simultaneous thermally activated delayed fluorescence (TADF) and phosphorescence processes on the basis of photophysical investigations and DFT calculations. Rather uniquely, simple manipulations in substituents of the PDP ligand and alteration of the terminal ligand allow fine-tuning of the ratio between TADF and phosphorescence from almost 100% TADF emission (Pt(MesPDPC6F5(BN)) to over 80% of phosphorescence (Pt(PhPDPPh(BuIm)). Apart from ratiometric capabilities, the complexes also are useful as decay time-based temperature indicators with temperature coefficients exceeding 1.5% K–1 in most cases. Immobilization of the dyes into oxygen-impermeable polyacrylonitrile produces temperature sensing materials that can be read out with an ordinary RGB camera or a smartphone. In addition, Pt(PhPDPPh)Py can be incorporated into biocompatible RL100 nanoparticles suitable for cellular nanothermometry, as we demonstrate with temperature measurements in multicellular colon cancer spheroids.


Computational Methods
All calculations were performed using the quantum chemistry package ORCA (version 5.0.2). 1 Visualization of molecular orbitals was done with Avogadro. 2Optimized geometries are provided as xyz files.
Stationary points were confirmed to be true energy minima by calculation of numerical frequencies.Subsequently, the structures were further optimized at triple-zeta level (def2-TZVP).All calculations were performed using the tight SCF settings and a denser grid (defgrid3).

2.2.Excited-state calculations
Excited-state calculations were performed using time-dependent density functional theory (TDDFT) as implemented in ORCA using the abovementioned method.2][13][14] Additionally, all S1 and T1 states have also been optimized.The influence of relativistic effects was also assessed for Pt( Ph IPDP Ph )(BN), Pt( Ph PDP Ph )(Py) and Pt( Mes PDP Ph )(BN) using the zeroth-order regular approximation (ZORA) 15,16 but was found to be negligible and was therefore not considered for the other structures.

Figure S9 .Figure S10 . 3 . 6 .
Figure S9.Photodegradation of platinum(II) complexes embedded in PS under anoxic conditions.The complexes where excited at the lowest energy absorption maxima for one hour with a xenon lamp of a Fluorolog-3 spectrometer.

1 .
Figure S16.Temperature dependence of (a, c, e) emission spectra and (b, d, f) green/red channel ratios of the photographic images for (a, b) Pt( Mes PDP Ph )(BN), (c, d) Pt( Ph PDP Ph )(BN) and (e, f) Pt( Ph PDP Ph )(BuIm) immobilized in PAN.The images in (b, d, f) show RGB images taken at the respective temperature.The spectra were recorded with a Fluorolog-3 spectrometer and the images were taken with an RGB camera (Sony Alpha 6000).

Figure S17 .
Figure S17.Normalized absorption spectra (black line) and emission spectra between 20 °C and 45 °C (5 °C steps, red and blue lines) of Pt( Ph PDP Ph )(Py) embedded in RL100-based nanoparticles, measured in aerated water.For recording the emission spectra, the particles were excited at 455 nm.

Table S1 .
Calculated HOMO and LUMO energies (E HOMO ,E LUMO ), energy gap between HOMO and LUMO (E gap ), energies of the first excited singlet and triplet state (E S1 , E T1 ), energy gap between first excited singlet and triplet (ΔE ST ) and dihedral angle (Θ) between the PDP ligand and the terminal ligand.

3.2.Luminescence decay in PS
Figure S8.Luminescence decay of platinum(II) complexes immobilized in PS under anoxic conditions at 25 °C.The decays were recorded at the emission maxima of the dyes.